1. Field of the Invention
The present invention relates generally to dispersion compensated optical waveguide fiber telecommunication systems, and particularly to such telecommunication systems that incorporate wavelength division multiplexing.
2. Technical Background
In optical waveguide fiber telecommunication systems designed to operate at high data rates over distances of the order of at least a hundred kilometers, compensation of total dispersion has been recognized as an efficient means of facilitating the system by reducing the number of electronic regenerators required. The concept of total dispersion compensation is, generally, the incorporation into the system of a compensating waveguide fiber having total dispersion sign opposite that of the primary transmission fiber. (The sign convention in common use is that dispersion of a waveguide is said to be positive if light of shorter wavelength travels faster in the waveguide than does light of longer wavelength).
Because wavelength division multiplexed systems can accommodate higher data rates over the same waveguide fiber, the concept of total dispersion slope compensation, i.e., compensation of total dispersion over a range of wavelengths, was introduced. Perfect compensation over a range of wavelengths can be achieved by selecting the total dispersion and dispersion slope of the compensating fiber to be the same multiple of the total dispersion and dispersion slope of a transmission fiber, respectively, while having signs opposite to those of the transmission fiber. This choice of total dispersion slope in the compensating fiber provides the capability of equally compensating the total dispersion of each of a number of signal wavelengths in the operating wavelength range of the system. That is, substantially equal total dispersion compensation is provided for each of the channels in the wavelength division multiplexed system.
Non-linear optical effects, which become important in these high performance telecommunication systems, can be mitigated through use of relatively higher effective area optical waveguide fiber. In certain dispersion compensated systems, both relatively high as well as relatively low effective area waveguide fibers are used. Non-linear effects can be held to a minimum by placing the lower effective area fiber at locations in the system where signal intensity is lower, so that non-linearity has less negative impact on system performance.
Recent investigations have focused on waveguide fiber combinations including large effective area transmission waveguide fibers used in conjunction with compensating waveguide fibers that compensate the total dispersion over a wavelength range (total dispersion slope compensation). Because the refractive index profile design of total dispersion slope compensating fibers does not in general include the property of large effective area, strategies have been developed in which transmission fiber and compensating fiber have been optimally placed to limit non-linear effects as well as compensate total dispersion over an operating wavelength range of the optical waveguide system.
An advantageous system or link configuration is one using a compensating fiber that has total dispersion and total dispersion slope related to the corresponding parameters in the transmission fiber by the same integral multiple. For example, a transmission fiber having total dispersion D and total dispersion slope S can be compensated by a fiber having total dispersion xe2x88x922D and total dispersion slope xe2x88x922S by using in the system a transmission fiber length to compensating fiber length ratio of 2 to 1 within each of multiple sections of the total transmission path. The system is said to be perfectly compensated because the ratio of total dispersion to total dispersion slope, xcexa, is identical for the fibers making up the system. A disadvantage of this scheme of perfect compensation is that the average total dispersion is zero for the compensated spans that together make up the waveguide transmission path for the system. When wavelength division multiplexed signals spend a significant amount of travel time in waveguide sections having zero or near zero total dispersion, the non-linear-effects cross phase modulation and four wave mixing can adversely affect system performance and increase the physical bit error rate.
One can effectively move away from the perfect compensation format using fibers having identical total dispersion to total dispersion slope ratio, by adjusting the length ratio of transmission to compensating waveguide fiber. However there are drawbacks in this system design because additional lengths of either the transmission fiber or the compensating fiber must be inserted into the system to remove accumulated total dispersion or accumulated total dispersion slope. In systems where average negative total dispersion is desired within each section or span, the compensating fiber is relatively longer than the transmission fiber. In effect, the lower effective area, compensating fiber is located in parts of the system where signal intensity is relatively higher, so that non-linear effects are more pronounced. In the alternative case, systems where average positive dispersion is desired, a long span of compensating fiber is needed to remove accumulated positive total dispersion and positive total dispersion slope, thereby introducing pulse distortion due to self phase modulation. An alternative to the use of the span of compensating fiber is the introduction into the span of a dispersion compensating module designed to compensate either total dispersion, total dispersion slope, or both. The drawback in this case is that additional optical amplifiers must be used to offset the signal attenuation in the dispersion compensating module.
There is therefore a need for a compensation format that provides for spans making up a system, wherein the spans have a net negative or net positive average total dispersion without incurring the penalties associated with prior art compensation schemes. The present invention addresses this need.
One aspect of the present invention is an optical waveguide fiber telecommunications link including a plurality of spans optically coupled in series, i.e., end to end. As used in this specification, an optical waveguide fiber link refers to the total length of waveguide fiber that propagates light from a light signal transmitter to a receiver. The term link will be understood to include components such as optical amplifiers, optical couplers, or wavelength division multiplexers used in telecommunication systems. Each span includes a first and a second optical waveguide fiber. Each of these waveguide fibers is characterized by a total dispersion (the sum of waveguide and material dispersion) at a particular wavelength xcex and by a total dispersion slope over a wavelength range, that includes xcex, of operation of the link. The total dispersion slope of the respective first and second optical waveguide fibers are selected to be opposite in sign. The magnitudes of the respective total dispersion slopes are selected to provide a particular accumulated total dispersion over the span at the wavelength xcex. Further, the respective total dispersion slopes are selected to provide, at the wavelengths over the operating wavelength range, accumulated total dispersion having a value within +/xe2x88x9210% of the value at wavelength xcex or +/xe2x88x9210 ps/nm, whichever is larger. That is, the end to end dispersion of a span, measured in units of ps/nm, exhibits the same value to within the larger of +/xe2x88x9210 ps/nm or +/xe2x88x9210% at each wavelength over the wavelength range. The span is said to be compensated over the wavelength range. Accumulated total dispersion, the dispersion experienced by a signal pulse traversing a length of waveguide fiber, is the product of fiber total dispersion times fiber length.
The total dispersion of each span is not completely compensated, i.e., total dispersion accumulates over length, because, in the instant invention, the respective products of total dispersion times length of the first and second fiber are not equal. The signs of the respective total dispersions of the first and second fibers are opposite to provide for partial-dispersion compensation of each of the spans. The respective values for ratio of total dispersion to total dispersion slope, xcexa, of the first and second fibers are not equal.
An operating wavelength range of the link which takes advantage of a low attenuation window of silica based optical waveguide fibers is from about 1480 nm to 1620 nm.
The average total dispersion of the link of the plurality of spans has an absolute value which lies in a range that does not include zero. This link average total dispersion can be defined in terms of span average total dispersion. The span average total dispersion is defined as the total dispersion of the first fiber multiplied by its length plus the total dispersion of the second fiber multiplied by its length, the sum (the accumulated total dispersion) then divided by the total span length. The average total dispersion of the link is the sum of the respective average total dispersion of each span making up the plurality of spans included in the link divided by the number of spans.
In another embodiment of this first aspect of the invention, the absolute value of the average total dispersion of the link has a range from 0.50 ps/nm-km to 4 ps/nm-km. An average total dispersion of the link in this range advantageously limits the non-linear phenomena four wave mixing and cross phase modulation.
Yet another embodiment in accord with the first aspect of the invention, includes a dispersion compensation module having zero total dispersion slope, and a total dispersion selected to compensate the accumulated total dispersion of the link. That is, the dispersion compensation module has accumulated total dispersion about equal in magnitude and opposite in sign to the accumulated total dispersion of the plurality of spans. In accord with the definition stated above, the accumulated total dispersion of the spans is the sum of length multiplied by total dispersion for each of the fibers making up the spans. The sum is taken over all the fibers making up the plurality of spans included in the link. The dispersion compensation module can be optically coupled essentially anywhere in the link. In cases where the effective area of the fiber in the compensation module is relatively small, the compensation module is coupled into the link at a point where the optical signal intensity is relatively low, i.e., the compensation module is spaced apart from the transmitters or amplifiers included in the link. An example of optimum placement of the compensation module is in the middle of a fiber length between a dual stage amplifier.
In an embodiment in accord with the first aspect of the invention, the first fiber of a span has positive total dispersion and the second fiber of the span has a negative total dispersion. The total dispersion of the second fiber has a magnitude of total dispersion less than that of the first fiber by an amount ranging from 2 ps/nm-km to 4 ps/nm-km. For example, the first fiber may have a dispersion of +19 ps/nm/km and the second a dispersion of xe2x88x9215 to xe2x88x9217 ps/nm/km. This configuration provides for local total dispersion along the link which is positive, an advantageous condition in terrestrial optical telecommunications systems. In a related embodiment, the negative total dispersion fiber in the span is greater in magnitude by a prescribed amount, for example, 2 ps/nm-km to 4 ps/nm-km. For example, the first fiber may have a dispersion of +19 ps/nm/km and the second a dispersion of xe2x88x9223 to xe2x88x9221 ps/nm/km. This configuration provides for local total dispersion along the link which is negative, a characteristic desired in certain systems, for example, in those in which soliton formation is to be suppressed or modulational instability should be reduced. In each of the two embodiments, the total dispersion slope of the fibers in a span are related to each other by a multiple factor in the range of 1 to 2. For a proper choice of respective lengths of the first and second fibers, the relationship between the respective total dispersion slopes allows for a compensation over the operating wavelength range in which a target total dispersion at each wavelength in the range can be achieved to within a desired tolerance, for example, +/xe2x88x9210%.
In a further embodiment of this aspect of the invention, the first and second optical waveguide fibers in each span have respective positive and negative total dispersion and total dispersion slope. The magnitude of the total dispersion of the second fiber in the span is less than twice the magnitude of the first fiber in the span by an amount in the range from about 2 ps/nm-km to 10 ps/nm-km. The magnitude of the total dispersion slope of the second fiber is related to the magnitude of the total dispersion slope of the first fiber by a multiple in the range of 1 to 2. In this configuration, the total length of the second fibers making up the link is less, but, by proper choice of the multiple, the total dispersion slope can be compensated to within a desired tolerance for each of the channels located within the link operating wavelength range. In a related embodiment, the total dispersion slope of the second fiber again is related to that of the first fiber by a multiple in the range of 1 to 2, and the total dispersion of the second fiber is greater than twice that of the first by an amount ranging from about 2 ps/nm-km to 10 ps/nm-km. The signs of total dispersion and total dispersion slopes are, respectively, positive and negative for the first and second optical waveguide fibers.
A particular advantageous choice of ranges of total dispersion and total dispersion slope for the first fiber in each of the plurality of spans is 17 ps/nm-km to 21 ps/nm-km and 0.05 ps/nm2-km to 0.08 ps/nm2-km, respectively.
In yet a further embodiment of the first aspect, a dispersion compensation module is optically coupled into the link at the input, i.e., the transmitter end, of the link. The dispersion compensation module is selected to have zero dispersion slope over the operating wavelength range of the link. The accumulated total dispersion of the module is selected to be opposite in sign to the accumulated total dispersion of the plurality of spans included in the link. This configuration provides an initial bias of the signal dispersion. Then, as the signal traverses the plurality of spans, the signal dispersion becomes closer to zero. Any residual accumulated total dispersion in the signal after traversing the dispersion compensation module at the input and the plurality of spans can be reduced to any pre-selected value, which can be chosen to be zero, by optically coupling an additional dispersion compensation module, having zero total dispersion slope and a pre-selected accumulated total dispersion, into the link.
The mode field diameter of the first fiber is in general not equal to that of the second fiber of a span, because the refractive index profiles of the two fibers are different. This mismatch of respective mode field diameters can result in unacceptably high splicing or connecting loss. To minimize such loss, a third fiber type may be used to serve as a splicing or connecting bridge (a bridge fiber) between the first and second fibers of a span or at other splices or connections within the link where the fibers to be spliced or connected are of unequal mode field diameter. Such a bridge fiber and the method using the bridge fiber is disclosed and described in application U.S. Ser. No. 60/303,302, entitled xe2x80x9cMethod of Connecting Optical Fibers, an Optical Fiber Therefor and an Optical Fiber Span Therefromxe2x80x9d, filed on Jul. 6, 2001 which is incorporated herein in its entirety by reference.
Thus an advantageous embodiment of the invention in accord with the first aspect of the invention and the other embodiments thereof disclosed and described herein further includes a bridge fiber at splices or connections between fibers in the link having a mismatch in mode field diameter.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.